The prior art is replete with electronic sensors, transducers, and circuits that detect or measure physical phenomena. For example, an accelerometer can be realized as a miniature sensor component that detects movement, changes in physical orientation, vibration, shaking, or the like. Miniature accelerometers can be included in a variety of applications such as mobile devices, portable video games, and digital media players. An accelerometer in such a device can be used to detect whether the display of the device is oriented in a portrait mode or a landscape mode, to transition between sleep and active modes, to obtain user input (e.g., shaking the device might represent a user command), etc.
Accelerometers in portable devices are often implemented with capacitive sensing cells. In this context, a capacitive sensing cell includes a plurality of capacitors that are arranged and energized such that the capacitance of the cell varies with its acceleration. In typical applications, the measured difference in capacitance is converted into a voltage that can be processed or analyzed in an appropriate manner. In particular, a capacitance-to-voltage interface circuit can be used to convert the measured capacitance differential into a corresponding analog voltage. However, due to the relatively low voltage levels associated with such capacitance-to-voltage conversion, the analog voltage is usually amplified to provide adequate dynamic range for purposes of subsequent analog-to-digital conversion.
Conventional capacitance-to-voltage interface circuits utilize a capacitive sensing cell and an amplifier stage (which may include one or more operational amplifiers) associated with the capacitance-to-voltage conversion and analog voltage amplification. In practice, a bank of selectable capacitors might be used to provide a desired offset voltage for the amplifier stage—the offset voltage is used to compensate for manufacturing variation and device tolerances to ensure proper operation of the interface circuit. Successive approximation register (SAR) based analog-to-digital converters (ADCs) also utilize a bank of capacitors that are used to determine the bit values corresponding to the analog voltage input.
Unfortunately, the offset capacitors and the SAR capacitors consume a significant amount of the physical space associated with such a capacitance-to-voltage interface circuit. Although a conventional implementation might function in an acceptable manner, the use of two or more distinct and separate capacitor banks is undesirable from both a physical packaging standpoint and a power consumption standpoint. In this regard, it is usually desirable to decrease the integrated circuit die area and, consequently, the overall physical footprint of mobile devices. In addition, it is usually desirable to decrease the electrical current requirements and, therefore, increase the battery life of mobile devices. Accordingly, it is desirable to reduce the physical size and power consumption of capacitance-to-voltage interface circuits.